I added some blue foam insulation to the interior my existing box and covered it with fiberglass panels from Lowes home center. The fiberglass panel is meant to be used on the wall of a cheap shower surround. It had a bumpy front surface, which is not what I wanted. But the back was smooth so I polished that and waxed it and used it as the inside of the fridge. On the other side I sanded the of the high points of the bumps and glued it to the insulation. I used caulk on the edges. It came out prettty well and was a cost effective way to upgrade.
There are 'pultruded' fiberglass panels that are probably a stronger solution, but they are also much more expensive. See White Fiberglass Sheet
for what rParts offers.
I used rParts to supply me with a new door - see RParts: Refrigeration Parts Solution
(Click down to building materials) I glued my own wood exterior onto one of these doors with thickened epoxy. The rood swings from hinges mounted to the glued on panel. These doors/hatches are also pretty expensive, but half the price at Rparts than I could find elsewhere.
Large door - $329 online; reaching the beer at the bottom of the fridge - priceless.